This invention relates to a control element for a power system stabilizer and more particularly to a digital filter included in a digital power system stabilizer coupled to the excitation circuitry of a power generation system.
In large power turbine generator installations, the utilization of fast responding electronic equipment for control of the generator field excitation has led to undesired stimulation of mechanical and electrical resonances. This problem has been found to be particularly severe when multiple generators are coupled electrically together or when long transmission lines are required to distribute the generated power.
This problem is generally overcome by the inclusion of a compensating control element called a power system stabilizer (PSS) in the feedback loop which regulates the generator output. A power system stabilizer is a well known combination of electrical circuitry which inserts an adjustable amount of phase lead into the field exciter circuit for compensating the destabilizing phase lags introduced in the power distribution system. The power system stabilizer has been conventionally implemented heretofore by analog techniques by using a multi-section analog filter whose characteristics are designed to provide the required gain and phase response. The multiple requirements that must be met by this filter are quite stringent, however. For example, the resonant frequencies of the undesired power system oscillations modes which must be suppressed are in a narrow band of frequencies approximately between 0.1 and 2 hertz (Hz).
As is well known, a dual lead/lag filter having adjustable parameters normally comprises the primary compensation element. In this relatively low frequency band, the needed phase characteristic leads to a rising gain characteristic as the frequency increases. The filter, however, must also at the same time yield a high attenuation in the frequency band from about 13.5 to approximately 45 Hz in order to avoid exciting the mechanical torsional resonances of the generator. Accordingly, a second filter, termed a torsional filter, is also included in combination with the first to provide a sharp drop in gain in this region. A high-order filter is needed to achieve this attenuation in the stop band but it tends to contribute significant additional phase lag at 2.0 Hz which cancels some of the desired phase lead. A third filter, comprising a notch filter, is also included to attenuate stray signals which appear in the control path at the fundamental alternating current (AC) power frequency, typically 60 Hz. In order to allow minor variations in the line frequency, this notch filter must be made relatively broad and thus can also contribute to undesirable phase lag.
Meeting all of the above mentioned requirements with an analog filter design is relatively difficult and requires the use of expensive components to achieve accurate pole/zero placement and low drift. The problem is further compounded by the need to make the primary lead/lag section adjustable in the field where the power generation installation is located.